Dielectric guide system



Feb, 1, 1933.

H. E. CURTIS DIELECTRIC GUIDE SYSTEM Filed April 11, I936 50000 IOOOOO5000 50000 20000 FRE OUENC Y ME 6.4 C YCLES 2 4 6 8 l0 l2 l4 INNERRAD/US OF CONDUCTOR CENT/MET l' IN VENTOR H. 5cm? r/s ATTORNEY PatentedFeb. 1 1938 UNITED STATES PATENT OFFEQE DIELECTRIC GUIDE SYSTEMApplication April 11, 1936, Serial No. 73,955

8 Claims.

This invention relates to systems for the trans mission ofelectromagnetic waves of ultra-high frequencies and more particularly tosystems for the transmission of such waves over dielectric guides.

It has been shown heretofore how electromagnetic waves of sufiicientlyhigh frequency may be transmitted through rods of dielectric material,metal-sheathed or otherwise, hollow conducting structures such asmetallic pipes, and other equivalent structures characterized by alateral boundary comprising an electromagnetic discontinuity.

The principal object of the present invention is to reduce theattenuation of the waves transmitted through a dielectric guide, or inanother aspect to reduce the efiective resistance of the guide todielectrically guided waves. A more specific object of the invention isto secure an optimum correlation between the frequency or frequencies ofthe guided waves and the transverse dimensions or other properties ofthe guiding structure.

The present invention is based on applicants discovery of the lawsgoverning the variation of the attenuation of dielectrically guidedWaves with frequency and the fact that at a certain frequency theattenuation of each of several different types of such waves is minimum.In accordance with the present invention, the frequency or frequencieschosen for the operation of a dielectric guide system is, or are, suchthat the attenuation is minimum, or if the operating frequency bepredetermined, the dimensions and physical properties of the guidingstructure are so related to that frequency as to result in minimumattenuation.

The nature of the present invention, together with other objects,features and advantages thereof, will appear more fully in the followingdiscussion and detailed description of several specific embodiments ofthe invention, reference being made to the accompanying drawing, in

which:

Figure 1 illustrates a typical dielectric guide system;

Figs. 2 and 3 show alternative terminal electrode structures;

Fig. 4 shows graphically the relation between attenuation and frequencyfor typical systems in accordance with Figs. 1, 2 and 3; and

Fig. 5 shows graphically the relation between the optimum frequency andthe dimensions of the guide for one specific type of wave.

Of the waves transmissible along a dielectric guide two general typeshave been identified: transverse magnetic and transverse electric. Thesetwo general types have been defined and illustrated in an applicationfor Letters Patent, 5 Serial No. 56,959, filed by S. A. Schelkunoff, onDecember 31, 1935. Transverse magnetic Waves, the first to be dealt withhere, are characterized by the fact that the vector representing themagnetic component of the wave lies substan- 10 tially wholly in a planeorthogonal to the direc tion of propagation, whereas the vectorrepresenting the electric component does not. Transverse electric wavesare characterized in a similar manner by the fact that the vector repre-15 senting the electric component of the Wave lies substantially whollyin a plane orthogonal to the direction of propagation, Whereas themagnetic component does not.

In Fig. l is represented a typical dielectric guide system adapted forthe long distance transmission of transverse electric Waves from agenerator or source I to a receiver 2. The guide illustrated comprises ahollow metallic structure 3, a copper tube, for example, evacuated orfilled with a fluid or solid-dielectric material, which may convenientlybe air. The source 5 is a generator of high frequency waves, and theseWaves may be modulated with speech, telegraph, television or othersignals to produce a wide 35 band of waves for application to themetallic terminal electrode structure 4 of the guide 3. At the receivingend of the system a similar electrode structure may be employed toconvert the dielectrically guided waves into conduction currentssuitable for operation of the detector or receiver 2.

Where the dielectric guide is a metallic tube, the attenuation of anytransverse magnetic wave in it is given by Equation 4 of the Schelkunoffapplication, supra:

a 1 where R1) is the mtrinsic resistance of the metal comprising thetube, the intrinsic resistance being the real part of its intrinsicimpedance;

and the other symbols employed have the following significance:

a=inner radius of the tube in cms. fc=cut-off frequency J appliedfrequency p.=intrinsic inductance of the dielectric in henries/cm.e=intrinsic capacity of the dielectric in farads/cm. V a =intrinsicinductance of the metal in henries/cm. g =intrinsic conductance of themetal mhos/cm. In empty space, a is 41r10 henries/cm. and e is 10- /361rfarads/cm. For copper, ,up is 41r10- henries/cm., and o is 5.800 x i0mhos/cm. The cut-off frequency fc to which reference has been made, isthe critical frequency above which dielectrically guided waves of agiven type are freely propagated along any particular dielectric guideand below which the attenuation is practically infinite. V V

For a metallic tubular guide of predetermined size and material,Equation 1 may be rewritten in the form:

where K is a constant.

To determine the frequency at which the attenuation is minimum,

E V or is equated to zero. Carrying out this operation and solving forthe optimum frequency fm,

f f cycles/sec. (5)

Equation 5 relates the optimum'frequency with the cut-off frequency ,fc.The latter is determinable, in turn, from the relation I f cycles/sec.,(6)

where v is the velocity of light in free space in centimeters per secondand k is the modular constant appropriate to the mode and order of theparticular dielectrically guided wave that is to be employed. A scheduleof the modular constants for transverse magnetic waves follows:

Transverse magnetic waves in circular tubes Mode Equations 5 and 6combine to yield the following expression for the optimum relationbetween the operating frequency fm and the internal radius a of thetubular guide:

'Fig. 2 shows a terminal structure comprising an axial metallic disk '5and a concentric metallic annular electrode 6 suitable for thegeneration and reception of transverse magnetic waves of zero order andfirst mode. This wave is of particular interest because it has a lowcutoff frequency, the lowest, in fact of all transverse magnetic waves.For this type of wave, Equation 7 reduces to: v

cycles sec. (8)

Equation 8 is shown in graphical form in Fig. 5..

Curve B of Fig. 4 shows the relation between frequency and theattenuation of this type of wave in a copper tube of eight-inch internaldiameter.

For specific example, suppose it be desired to determine the optimumfrequency at which to transmit waves of the character last described ina metallic tube of four-inch inner radius. From 7 Equation 8 or fromFig. 5, it may readily be foundthat the optimum frequency isapproximately 1960 megacycles per second. If the waves 'occupy a widefrequency band, the band should be roughly centered about the optimumfrequency.

It has been tacitly assumed that the dielectric enclosed by the metallictube is substantially gaseous. If it is not substantially gaseous, theeffect is twofold. First, the cut-off frequency In is decreased in theratio of the square root of the dielectric constant of the dielectricmaterial referred to air as unity. Second,,the attenuation is increasedby losses in the dielectric material, these losses being in general acomplex function, of frequency. In such cases it is necessary todetermine the dielectric loss experimentally, combine it with thecomputed attenuation and then solve graphically for the optimumfrequency.

Transverse electric waves remain to be considered. Fig. 3 shows aterminal structure comprising two radial ccnductors 1, suitable for thegeneration of one type of such waves. This same structure is adaptedalso for the receiving end of a dielectric guide system in the mannerillustrated in Fig. 1. first order and first mode may be generated withthe structure shown in Fig. 3, and curve C of Fig. 4 shows how theattenuation of this type of wave varies with frequency in a copper tubeof eight-inch internal diameter.

The attenuation of transverse electric waves of order n may beconveniently expressed in sub stantially the same form as Equation (4.1)of Schelkunoff, supra.

Jl a na k n where k is the modular cr nstant appropriate. to the orderand mode of the wave. the modular constants for transverse electricwaves follows:

Transverse electric waves in circular tubes Mode H Orde To determine theoptimum operating frequency,

Transverse electric waves of.

A schedule of.

or, substituting from Equation 6,

where all factors on the right-hand side of the equation are known assoon as the inner radius of the tubular guide and the type of wave havebeen fixed upon.

As a typical example, assume that a metallic tube with air dielectric isto be used for the transmission of transverse electric waves of thefirst mode and first order, and that the tube has an inner radius offour inches. The value of k for this particular wave is found from theschedule of modular constants to be 1.84. The cut-off frequency is foundfrom Equation 6 to be 865 megacycles per second. The frequency at whichattenuation is minimum is then found from Equation 11 to beapproximately 2750 megacycles per second.

The principles of this invention can be extended to the determination ofthe optimum frequency for transmission in hollow conductors ofnoncircular cross-section since it is evident that waves of transverseelectric and magnetic types can be sustained therein. Thus the guide maybe rectangular, elliptical or otherwise in cross-section. The cut-offfrequencies are given by modular constants which are obtained byapplying boundary conditions to the mathematical equations which expressthe field as in Schelkunoff, supra. Thus there will be a series ofcut-off frequencies, each series being a function of the cross-sectionalshape of the guide. The attenuation formulas also are dependent on thecross section of the conductor but the frequency for which theattenuation of a particular wave traveling along a tube of particularcross-section is a minimum can be obtained as has been done above forthe tubes of circular cross-section by a process of mathematicalminimization. The frequency for minimum attenuation will be found ineach case to be a function of the cut-off frequency.

What is claimed is:

1. A hollow metallic guide and means for establishing thereindielectrically guided waves, the frequency of said waves being sorelated to the transverse dimensions of said guide that attenuation issubstantially minimum.

2. A combination in accordance with claim 1 in which said waves aretransverse magnetic and the frequency is /313.- where fc is the cut-offfrequency of said guide.

3. A combination in accordance with claim 1 in which said waves aretransverse electric and the frequency is /3lcfc/n in accordance withEquation 11.

4. A wave guide comprising a dielectric medium and metallic meansdefining the lateral boundary thereof, and means for establishing withinsaid guide electromagnetic waves of a character such that there is atransmission cutoff frequency dependent on the transverse dimensions ofsaid guide and the index of refraction of said medium, the frequency ofsaid waves being higher than said cut-off frequency and so relatedthereto that the attenuation of said waves is substantially minimum.

5. A wave guide consisting essentially of a metallic pipe enclosing adielectric medium, and means for establishing progressiveelectromagnetic waves in said dielectric medium, the attenuation of saidwaves being a function of frequency and the frequency of said wavesbeing such that the attenuation is substantially minimum.

6. In a high frequency transmission system, a wave guide comprising ametallic pipe, and means for transmitting through the interior of saidpipe electromagnetic waves of a character such that there is a criticalfrequency separating the propagation range from a lower frequency rangeof zero or negligible transmission, the frequency of said waves beingapproximately that for which the attenuation is minimum.

7. In combination, a wave guide comprising a metallic pipe, and meansfor establishing in said pipe intelligence-bearing electromagnetic wavesof the transverse magnetic type, for which there is a cut-off frequencyfunctionally related to the transverse dimensions of said pipe, saidwaves occupying a wide frequency band including a frequency that is ofthe order of /3 times the said cut-off frequency, whereby said waves aretransmitted with low attenuation.

8. A transmission system including as a wave guide a metallic pipecarrying within it high frequency electromagnetic waves of thetransverse electric type such that said guide exhibits a high-passfilter characteristic, the frequency of said waves and the transversedimensions of said pipe being so related that the attenuation issubstantially minimum.

HAROLD EVERDELL CURTIS.

